1. Field of the Invention
The present invention relates to a double pipe exhaust manifold capable of preventing spatter from mixing into an exhaust gas, which is sucked from an inner pipe of the double pipe exhaust manifold, to a connection pipe connecting to an EGR valve.
2. Description of the Related Art
Generally, the exhaust manifold has a double-pipe structure including an inner pipe 101 and an adiabatic outer pipe 102 surrounding the inner pipe, as shown in FIG. 8.
The outer pipe 102 has two pipe members, which are formed so that the outer pipe is divided into two members 102a and 102b (hence, those pipe members will frequently be referred to as “divided pipe members”), and over lapping parts 102c and 102d of those divided outer pipe members 102a and 102b are placed one upon the other and welded by welding 102e to thereby form an integral pipe construction.
Normally, connected to the double pipe exhaust manifold is a connection pipe which is connected to an exhaust gas recirculation valve (referred to as an EGR valve) which returns part of the exhaust gas to a suction system of the engine. For the connection of the connection pipe connecting to the EGR valve to the double pipe exhaust manifold, two connection methods are known. A first connection method is that the connection pipe connecting to the EGR valve is directly connected to the double pipe exhaust manifold. A second connection method is that the connection pipe connecting to the EGR valve is connected to a connection member mounted on an exit of the double pipe exhaust manifold.
In the first connection method, as shown in FIG. 9A, opening holes 104 and 105 are formed in the inner pipe 101 and the outer pipe 102, respectively, while being successively arranged in the radial direction. A connection pipe 103 connecting to the EGR valve is connected to the double pipe exhaust manifold in a state that it is communicatively connected to the opening hole 105. In such a construction, part of the exhaust gas passing through the inner pipe 101 is sucked into the connection pipe 103 connecting to the EGR valve, by way of both the opening holes 104 and 105.
In the second connection method, as shown in FIG. 9B, a connection member 106 to which the connection pipe 103 connecting to the EGR valve is connected is mounted between the exit of the double pipe exhaust manifold and catalyst (not shown). In the structure, part of the exhaust gas passing through the inner pipe 101 is sucked into the connection pipe 103 connecting to the EGR valve. Incidentally, in FIG. 9, reference numeral 100 designates space retaining members used for securing an adiabatic space between the inner pipe 101 and the outer pipe 102.
As described above, the double pipe exhaust manifold according to the related art has such a structure that the overlapping parts 102c and 102d of the divided outer pipe members 102a and 102b are placed one upon the other and are formed integrally by welding. When the overlapping parts 102c and 102d of the divided outer pipe members 102a and 102b are welded together, spatter 107 is scattered into the outer pipe 102 through a gap between the overlapping parts 102c and 102d, and sticks to the outer side of the inner pipe 101 and the inner side of the of the outer pipe 102.
The spatter 107 may be removed to some amount from the pipes by striking the pipes with a wood hammer after the welding. In this case, however, it is impossible to completely remove the spatter from the pipes. Accordingly, the double pipe exhaust manifold is assembled to the vehicle body in a state that part of the spatter 107 remains in an annular space 101a between the inner pipe 101 and the outer pipe 102.
When the inside of the manifold is heated, by the exhaust gas, to be high in temperature during the engine operation, the residual spatter 107 is molten and peeled off by heat and vibration. The spatter mixes into the exhaust gas as sucked from the inner pipe 101 into the connection pipe 103 connecting to the EGR valve, and will strike the EGR valve to deform a shaft of the valve, or will enter the valve to thereby hinder the exact operation of the EGR valve.
The EGR valve of the electronically controlled type which is small in size and light in weight, currently prevails. This type of the EGR valve is sensitive to a very small amount of spatter to possibly operate erroneously or be damaged.
Further, the spatter 107 having passed through the EGR valve enters the suction system of the engine and to the interior of the engine to possibly cause an engine trouble.